1. Field of the Invention
The present invention is directed to a method for carrying out chemical equilibrium reactions using a permselective membrane. The invention is directed particularly to a method for converting carbon dioxide and hydrogen using a carbon membrane.
2. Discussion of Background Information
The conversion of hydrogen with carbon dioxide is pursued essentially for two reasons. On the one hand, hydrogen is increasingly produced by electrolysis for buffering grid fluctuations from the increasing proportion of fluctuating flow from regenerative sources (“power to gas”). The storage and distribution of hydrogen can be carried out directly in the natural gas network in small proportions. Storage and distribution would be possible on a much larger scale by converting with carbon dioxide to methane. The second reason relates to the chemical bonding and utilization of carbon dioxide and, therefore, has considerable advantages in technical respects relating to environment and climate.
In the Sabatier reaction, which is a chemical equilibrium reaction and is known to the person skilled in the art, carbon dioxide (CO2) and hydrogen (H2) (reactants) are converted to methane (CH4) and water (H2O) (products). The Sabatier reaction is usually carried out at pressures of ˜1 . . . 3 bara (absolute pressure; bar, absolute), also infrequently at higher pressures of up to 15 bara, and in a temperature range of around 200 to 450° C. Known suitable catalysts for this reaction are, for example, nickel, ruthenium, rhodium and cobalt which are usually arranged on an oxide support such as SiO2, TiO2, MgO, Al2O3 and La2O3 as is described, for example, in Ohya et al., 1997: Methanation of carbon dioxide by using membrane reactor integrated with water vapor permselective membrane and its analysis, Journal of Membrane Science 131 (1-2), the entire disclosure of which is incorporated by reference herein. The most successful combination at the present time is that of 0.5% Ru over TiO2. Depending on the catalyst, high CO2 conversions and CH4 yields are achieved at temperatures of ˜300 . . . 350° C. (John Ralston (www.pennenergy.com/index/power/display/0723256773/articles/pennenergy/ugc/renewable/the-sabatier-reaction.html).
The Sabatier reaction is a very demanding reaction because it is highly exothermic and, depending on the degree of dilution with inert gases, leads to temperatures above 600° C. after the addition of the reactants. At this temperature level, not only are some catalysts destroyed—for example, active sites are lost, e.g., by sintering of Ni particles in the nm range—but the reaction equilibrium also shifts in the reactant direction. On the other hand, temperatures that are too low result in kinetic limiting of the reaction and amount of the desired products.
Various publications have already addressed the use of membranes. Accordingly, the use of a membrane of twenty-coat hydrophilic porous glass on a ceramic support results at best in an 18-percent increase in CO2 conversion (at 300° C., 0.2 MPa and space velocity=0.0308 s−1) (Ohya et al.). The conversion increase is noteworthy, but the space velocity ranges far below required industrial ranges of at least 3 s−1. Moreover, concentration of carbon dioxide by the membranes precedes the actual Sabatier reaction (Hwang et al. 2008: A membrane-based reactive separation system for CO2 removal in a life support system, Journal of Membrane Science 315 (1-2): 116-124, the entire disclosure of which is incorporated by reference herein Calculations show synergistic positive effects for the reaction in this case. However, further process concepts are also possible. The hydrogen conversion under consideration here can be increased by approximately 90% to up to 99% over the procedure of reaction with two reactors connected in tandem with intermediate removal of the reaction water (Habazaki et al. 1998: Co-methanation of carbon monoxide and carbon dioxide on supported nickel and cobalt catalysts prepared from amorphous alloys, Applied Catalysis A: General 172 (1): 131-140, the entire disclosure of which is incorporated by reference herein). However, the separation of the products methane and water is a mandatory prerequisite for possible injection into the natural gas grid.
Since the Sabatier reaction is an exothermic equilibrium reaction, it is limited with respect to yield as reaction temperature or process temperature increases. Removal of a reaction product directly from the reaction in the process causes an increase in the driving force for product formation. A suitable water-separating membrane achieves precisely this in that the reaction product water is separated directly in the process. Accordingly, yield is increased compared to the process without membrane. Further, by separating out the reaction water, subsequent drying of the methane to ensure feed quality is superfluous and this process step can be omitted.
Methanol can be produced, for example, from a syngas of carbon monoxide and hydrogen. Particularly in the production of methanol from carbon dioxide and hydrogen as reactants and methanol and water as products of a chemical equilibrium reaction, the yield can likewise be improved when it is possible to separate the water on the product side.
In order to separate water from gas mixtures at elevated temperatures, it is conventional to use hydrophilic membranes. These hydrophilic membranes are, in particular, SiO2 membranes and zeolite A membranes (Wang et al., 2011: “Effects of water vapor on gas permeation and separation properties of MFI zeolite membranes at high temperatures, AlChE Journal 58(1); Sano et al., 1994: “Separation of ethanol/water mixture by silicate membrane on pervaporation, Journal of Membrane Science 95 (3); Hamzah et al., 2013: “Pervaporation through NaA zeolite membranes—A review”, The Third Basic Science International Conference 2013, the entire disclosures of which are incorporated by reference herein).
The preferred application is dewatering and drying of solvent vapors. However, the use of these membranes in the Sabatier reaction for separating water from a mixture with hydrogen, carbon dioxide and methane shows an insufficient selectivity and, further, an insufficient stability of the materials under the required conditions for the Sabatier reaction. Under reducing conditions such as are present in the Sabatier reaction, SiO2 membranes are subject to thermal degradation above 100° C. (Damle at al., 1995: “Thermal and chemical degradation of inorganic membrane materials”, Technical Report; Gu et al.: “Hydrothermally stable silica-alumina composite membranes for hydrogen separation”, Journal of Membrane Science 310 (1-2): 28-37), and zeolite A membranes have insufficient stability in acidic environments and sometimes limited thermal stability (Hamzah et al., 2013: Pervaporation through NaA zeolite membranes—A review”, The Third Basic Science International Conference; C03; Caro et al., 2009: Why is it so extremely difficult to prepare shapeselective Al-rich zeolite membranes like LTA and FAU for gas separation?, Separation and Purification Technology 66(1): 143-147, the entire disclosures of which are incorporated by reference herein) such as those occurring in combinations of H2O and CO2.
It would be advantageous to have available a method of carrying out equilibrium reactions in which water is separated on the reaction product side and an improved yield of reaction products is achieved.